1. Field of the Invention
This invention relates to a liquid crystal panel which is capable of displaying a high quality image in both modes consisting of a reflection mode which uses outdoor daytime light as an illumination lighting source and a transmission mode which uses a backlight incorporated in the panel as an illumination lighting source, and a liquid crystal display device which uses this liquid crystal panel.
2. Description of the Related Art
A liquid crystal display device which is used as display means of a personal computer or a monitor of other devices is a device which irradiates an illumination light to an image formed on a liquid crystal panel and emits a transmitted light or a reflected light to the display surface side so as to visualize the image.
That is, this type of liquid crystal display device, generally, uses a liquid crystal panel which sandwiches a liquid crystal layer in a space defined by adhering a pair of substrates having pixel selecting electrodes and the like, and forms an image by changing the orientation direction of liquid crystal molecules of selected pixel portions. The formed image per se is not in a visible condition so that the liquid crystal display device is constructed such that light is given from the outside and is irradiated to the liquid crystal panel and the transmitted light or the reflected light is observed.
This type of liquid crystal panel is classified into a transmission mode which uses a light source mounted behind the liquid crystal panel, or a so-called backlight and a reflection mode which uses outdoor daytime light which is present in the surroundings of the liquid crystal panel.
Furthermore, its power source capacity is limited. For example, with respect to PDA (small-sized portable data terminals), since their respective power sources per se mounted thereon are small, most of these terminals have a reflection display mode function which takes in surrounding light as an illumination light while excluding an active light source of the liquid crystal panel such as a backlight as an illumination lighting source. However, to assure the use of the liquid crystal panel in an atmosphere where the surrounding light is small or there is no outdoor daytime light at all, several liquid crystal display devices which are provided with auxiliary light sources have been manufactured to assure the use of the liquid crystal panel even in a dark environment.
This transmission and reflection type liquid crystal panel (half reflection type liquid crystal panel) which can be used in both the reflection mode and the transmission mode is provided with a half reflection plate on an outside surface of a substrate (lower substrate) opposed to a display surface side substrate (upper substrate). In such a liquid crystal panel, the outdoor daytime light which enters through the observation side substrate as an incident light is reflected and thereafter is emitted from the upper substrate so as to visualize an image, and the light irradiated from the backlight disposed behind the liquid crystal panel is transmitted through the lower and upper substrates and is emitted from the upper substrate so as to visualize the image.
FIG. 6 shows a cross sectional view schematically explaining the construction of the conventional half reflection type liquid crystal panel. The panel is constructed such that a liquid crystal layer 7 is interposed between a lower substrate 1 made of a glass plate or the like which is provided with a lower transparent electrode 4 on an inner surface thereof and an upper substrate 5 made of a glass plate or the like which is provided with a lower transparent electrode 6 on an inner surface thereof, the periphery of the liquid crystal layer 7 is sealed by a seal member 8, a lower polarizing plate 9 and a half reflecting plate 2' are laminated on the surface of the lower substrate 1, and a retardation film 10, an upper polarizing plate 11 and a light diffusing plate 15 are laminated on the surface of the upper substrate 5.
Although FIG. 6 shows a liquid crystal panel for a monochromatic display, the panel can be converted to the one for a color display by merely providing three color filters to the inner surface of the upper substrate or the lower substrate.
The upper polarizing plate 11 and the lower polarizing plate 9 which are disposed at the upper and lower portions of the liquid crystal panel have a function to emit the modulated light which transmits through the liquid crystal layer 7, that is, the light which has its orientation direction controlled at selected pixels toward the display surface side. The retardation film 10 corrects a thickness of respective substrates and the liquid crystal layer of the liquid crystal panel and .DELTA.nd (retardation) of other constitutional members. Furthermore, the light diffusing plate 15 prevents lowering of the brightness by scattering light emitted from the liquid crystal panel while preventing the reflection of the outdoor daytime light.
With the type of the liquid crystal panel shown in FIG. 6, in the reflection mode which uses the outdoor daytime light as the illumination light, the light entered from the upper substrate 5 side which defines the display surface side is transmitted through the lower substrate 1 and is reflected by the half reflection plate 2' and again is transmitted through the lower substrate 1 and the upper substrate 5 and is emitted toward the display surface side.
On the other hand, in the transmission mode which uses the backlight as the illumination light, the light irradiated by the backlight (not shown in the drawing) disposed at the rear side of the liquid crystal panel, that is, on the rear surface of the half reflection plate 2' is transmitted through the half reflection plate 2', the lower substrate 1 and the upper substrate 5 and is emitted toward the display surface side.
In case there is the outdoor daytime light but its level of brightness is low, the display is performed using the transmission-and-reflection mode where the backlight is turned on so that the reflection mode and the transmission mode are both used.
Furthermore, in the PDA and the like, there is a type which is provided with a so-called touch panel (not shown in drawings) for directly inputting data or the like from the display surface using a pen, a finger or the like. Such a touch panel is laminated above the upper substrate of the liquid crystal panel and upon pressing the touch panel with a nib, given information is directly inputted from a screen, is displayed on the liquid crystal panel, is stored in memory means incorporated in the touch panel, and is used for the reproduction.
With respect to the above-mentioned conventional liquid crystal display device, however, in the reflection mode, for example, the outdoor daytime light reaches the half reflection plate after passing through the upper substrate and the lower substrate and the light reflected by this half reflection plate again passes through the lower substrate and the upper substrate and is emitted toward the display surface side as the outgoing light and hence, the utilization efficiency of the outdoor daytime light used as the outgoing light is low so that there arises a problem that it is difficult to enhance the brightness of the display screen.
Furthermore, in the transmission mode, the light irradiated from the backlight suffers from a considerable loss at the half reflection plate so that it is difficult to obtain the sufficient brightness and the enhancement of the contrast is also limited.
Still furthermore, in the transmission mode, there has been a problem that the sufficient brightness is not obtained when the phase difference exists between the phase of the reflected light of the outdoor daytime light emitted after being reflected by the half reflection plate and the phase of the transmitted light which is emitted after passing through the half reflection plate. Furthermore, with respect to the color display, in the reflection mode particularly, there exist problems such that the light advances through the lower substrate and then returns through the lower substrate so that the deterioration of chroma occurs and the color mixture occurs due to parallax.
Accordingly, it is an object of the present invention to provide a half reflection type liquid crystal panel capable of displaying the image having high brightness and high contrast and a liquid crystal display device using such a liquid crystal panel.